In recent years, to meet demands for batteries for use in vehicle installation and demands for shift to direct current in large size devices out of consideration for environmental issues, the development of a compact and light secondary battery which is capable of fast charge and high current release has been desired. As a typical battery which meets such demands, there is a nonaqueous electrolyte secondary battery in which a lithium intercalation compound obtained by making carbon serving as a host material (which is herein a material capable of absorbing and releasing lithium ions) absorb an active material such as lithium metal, lithium alloy or the like, or lithium ions is used as a negative electrode active material and an aprotic organic solvent in which lithium salt such as LiClO4, LiPF6 or the like is dissolved is used as an electrolyte.
The nonaqueous electrolyte secondary battery includes a negative electrode in which the negative material is held by a negative electrode current collector serving as a support body, a positive electrode in which a positive electrode active material undergoing electrochemical reaction in a reversible fashion is held by a positive electrode current collector serving as a support body, and a separator which holds an electrolyte and is provided between the negative electrode and the positive electrode to prevent short-circuit from occurring in each of the electrodes.
The positive electrode and the negative electrode, each of which is formed into a sheet or a foil, are stacked in order with the separator interposed therebetween or are spirally wound with the separator interposed therebetween to serve as a heat generation element. Then, the heat generation element is stored in a battery case made of metal such as stainless steel, nickel plated iron, aluminum or the like. Then, after injecting an electrolyte into the battery case, a lid plate is sealed and fixed on an opening end portion of the battery case. Thus, a battery is formed.
A lithium ion secondary battery might possibly overheat due to electrical factors such as overcharge, environmental factors such as high temperature exposure, and mechanical factors such as damages caused by fall of a heavy load, and the like. As a method for testing whether a battery overheats due to mechanical factors, there is a method in which, assuming a most severe situation, besides dropping a heavy load onto the battery and adding vibration to the battery, an internal short-circuit is generated by sticking a nail (φ5) in a charged lithium ion secondary battery (in this method, an abuse stated in the SBA standard, in which a nail or the like is mistakenly stuck in a battery when the battery is packaged in wooden case is assumed). Results of tests conducted in such severe situations show that overheating of a lithium ion secondary battery is possibly caused in such situations.
As means for preventing overheating of a battery due to electrical factors or mechanical factors, a method in which an electric resistance of an active material is increased has been proposed (see, for example, Patent Document 1). Specifically, a method in which lithium-cobalt composite oxide of which a resistance coefficient when powder filling density is 3.8 g/cm is 1 mΩ·cm or more and 40 mΩ·cm or less is used as a positive electrode active material has been proposed.    Patent Document 1: Japanese Laid-Open Publication No. 2001-297763